Reference is made to commonly assigned U.S. Pat. No. 6,394,870 filed Aug. 24, 1999 by Dwight J. Petruchik et al., and U.S. patent application Ser. No. 09/723,389, filed Nov. 28, 2000 by David M. Johnson et al., the disclosures of which are incorporated herein by reference.
The present invention relates to providing an ultraviolet blocking layer for polymer-dispersed liquid-crystal displays.
Currently, information is commonly displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information. Magnetically written data, however, is not visible.
A structure is disclosed in PCT/WO 97/04398, entitled xe2x80x9cElectronic Book With Multiple Display Pagesxe2x80x9d which is a thorough recitation of the art of thin, electronically visibly written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a xe2x80x9cbook,xe2x80x9d each sheet arranged to be individually addressed. The patent recites prior art disclosing thin, electronically written pages in the form of flexible sheets, including image-modulating material formed from a bistable liquid crystal system and thin metallic conductor lines on each page.
Fabrication of flexible, electronically written display sheets are also disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITO conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar(copyright) polyester. A dispersion of liquid-crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid-crystal material. Electrical potential applied to opposing conductive areas operate on the liquid-crystal material to selectively expose display areas. The display uses nematic liquid-crystal material which ceases to present an image when de-energized.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a polymer-dispersed chiral-nematic liquid crystal, also referred to as polymer-dispersed cholesteric liquid crystal. The chiral-nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light-scattering focal-conic state. Said structure has the capacity of maintaining either one of the given states in the absence of an electric field.
U.S. Pat. No. 5,539,552 recites failures of liquid-crystal displays due to ultraviolet light affecting the liquid-crystal material, seals and thin films in displays. The patent discloses a solution for these problems by incorporating a glass plate that is compatible with the display, approximately 36 mils thick. The resulting display blocks 99 percent of ultraviolet light energy with wavelength less than 400+/xe2x88x9210 nanometers. Such plates, however, are inflexible, heavy and expensive.
It would be desirable to provide a method of providing a polymer-dispersed cholesteric display with protection from ultraviolet (xe2x80x9cUVxe2x80x9d) radiation without requiring the presence of glass plates. It would be desirable to provide UV protection for a display having a thin flexible substrate.
It is known that fine particles of titanium dioxide in a binder can provide a visually transparent ultraviolet blocking layer that blocks ultraviolet radiation. U.S. Pat. No. 5,736,308 discloses a method in the photographic arts to produce titanium dioxide particles that are transparent to visible light and absorbent to UV light of a broad spectrum. Titanium dioxide (TiO2) pigment having an average size from 0.02 to 0.1 microns is formed, which exhibits good absorption from 290 to 400 nanometers while being functionally transparent in the visible region. U.S. Pat. No. 5,736,308 discloses a multilayer photographic film with a UV protective layer using fine titanium dioxide particles. The TiO2 particles are dispersed in xe2x80x9cgelatinexe2x80x9d (gelatin). A considerably thinner layer is achieved than with customary organic UV blocker dyes that are dispersed in oil droplets.
Another problem in the prior art relates to the manufacture of displays made by coating layers of materials to form polymer-dispersed liquid-crystal display sheets. Polymer-dispersed liquid-crystal layers can be vacuum coated with conductive metal which is then laser etched to form an image-forming electrode. The use of laser etching can be problematic in that the UV laser that produces etching in one layer of the structure has the potential to damage the other layers, for example another conductor layer, in the structure, typically an intermediate, on an assembly line, formed in the manufacture of a display. This potential problem can limit, render difficult, or even prevent the use of desired or optimal laser-etching techniques
Thus, it would be useful and advantageous to impart UV blocking, in this case blocking of a UV laser, into the intermediate structures used to make display sheets. For example, it would be advantageous to provide UV blocking to protect a first conductors not intended to be etched during a laser etching process involving a second or different conductor.
In view of the above, it would be desirable to either protect a polymer-dispersed liquid-crystal display from UV radiation during use and/or to protect a polymer-dispersed liquid-crystal display, or intermediate thereof, from UV radiation during manufacture.
It is the main object of this invention to protect polymer-dispersed liquid-crystal displays from ultraviolet light during use. In another aspect of this invention, it is possible to protect the same or manufacturing intermediates thereof from ultraviolet radiation from an ultraviolet laser used for etching of conductors during manufacture. This invention can be used to protect second conductors in the display from ultraviolet radiation during laser etching of first conductors. It is possible for the same means that provides a polymer-dispersed liquid-crystal display with ultraviolet protection during use to also function to protect the display, or manufacturing intermediate thereof, from ultraviolet radiation during manufacture.
These objects are achieved by a display comprising:
a) a substrate;
b) transparent first conductors;
c) second conductors;
d) a layer comprising polymer-dispersed liquid-crystal material disposed between the first and second conductors;
e) at least one barrier layer comprising a polymeric binder and water-dispersible or water-soluble ultraviolet blocking compounds or particles, which barrier layer is disposed to block ultraviolet radiation from striking said polymer-dispersed liquid-crystal material.
In one embodiment of a display according to the present invention, the barrier layer comprises ultraviolet-blocking inorganic particles in a binder. Fine particles such as titanium dioxide and/or other inorganic ultraviolet-absorbing particles can be incorporated in the barrier layer of the display. For example, a low concentration of TiO2 particles can block UV radiation. Alternatively, a ultraviolet-blocking dye can be used, preferably contained in a discrete organic phase within a continuous aqueous phase in the barrier layer. Such UV-blocking materials are chemically and electrically inert. Thus, the barrier layer can be located between the electrodes of the display without affecting display performance.
The invention is also directed to a method of manufacturing the above display.